Both his early life and the details of his career as Bishop of Dunblane are not well known, however it is known that he held the latter bishopric between 1403 and his death in 1419.
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He is last known to have been alive on 25 March 1419 but was dead by 30 October when a papal mandate ordered the translation of William Stephani, Bishop of Orkney, to the now vacant see of Dunblane.
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Some time after the death of Bishop Dúghall (last attested 1398 × 1401), Fionnlagh was elected as Bishop of Dunblane; on 10 September 1403, Fionnlagh was provided to the bishopric by the Pope directly, the election being illegal due - so it was claimed - to earlier papal reservation of the see.